1. Field of the Invention
The present invention relates to an automatic heating apparatus employing a gas sensor and a weight sensor to detect the condition of an object to be heated.
2. Description of the Prior Art
Conventionally, such an automatic heating apparatus that is designed to automatically control the heating time of food has been widely put into practical use. An automatic electronic oven is one example of such an apparatus, and is highly regarded in terms of convenience, and accordingly occupies a considerably large share of the oven market. The above-described automatic heating apparatus embodies various types such as one equipped with a gas sensor which reacts to steam or various kinds of gases generated during heating of the food, or an infrared ray sensor for detecting the surface temperature of the food, or a thermistor for detecting the temperature of the air flowing in and out of a heating chamber. The heating operation is designed depending on the kind, condition or the like of the food to be heated in any one of the above-described types. For example, in the heating apparatus marketed during the first stages of development of these apparatus, several select keys are employed in general for selecting the reheating operation. A representative example is U.S. Pat. No. Re. 31,094. FIG. 1 is a perspective view of such a prior art apparatus as referred to above. FIG. 2 is a front view of an operating panel of the prior art apparatus of FIG. 1. In the heating apparatus of FIG. 1, a door member 2 is so provided as to be freely opened or closed in the front face of a main body 1. The apparatus has many select keys 4 arranged on an operating panel 3 in the front face of the main body 1 for selecting the type of heating operation depending on the temperature condition or the kind of food. In the category of food for reheating, five select keys, namely, for "cold boiled rice", "soup", "curry/stew", "frozen boiled rice" and "frozen curry" are provided. The reason for the provision of the five select keys resulted from the fact that, if the heating operation were stopped at the time when the steam or gas was generated from the food, or when the surface temperature of the food had reached a predetermined temperature, some kinds of food would not have been heated sufficiently at the center thereof, requiring further heating, the time for which however varies for each type of food. FIG. 3 shows the relationship of the heating time and the amount of the steam generated from the food as detected by the gas sensor. In the graph of FIG. 3, T1 is the time prior to a first detection point when a predetermined amount of steam is detected by the gas sensor. When cold boiled rice is reheated, it is good to stop the heating thereof at a time at which the generation of steam is detected. In the case of reheating soup, it is still lukewarm at T1 without being heated further for an additional time K1.times.T1, K1 being constant and selected to be 0.1-0.5 from experience. In the case of jelly-like curry or stew, it is necessary to heat the same over a time K2.times.T1 in addition to the time T1, K2 being 0.3-0.8. For frozen rice with little moisture which is obtained by freezing cold boiled rice, it is necessary to reduce the heating caloric value from the time point T1 when the steam is detected, and heat the rice over a time K3.times.T2 in addition to the time T2 prior to the second detection point when a predetermined amount of steam is detected from the food after the entirety of the frozen food is defrosted, K3 being 0.01-0.5. Also, for frozen curry with much moisture, which is obtained by freezing cold curry, it can be heated to a suitable temperature if the curry is fully defrosted, with the heating caloric value reduced from the time point T1, and is further heated over a time K4.times.T2 after the time point T2 when the predetermined amount of steam is detected, K4 being 0.3-2.0. The reason why the value of K is different for each food is that the steam is generated in a different way from each food, and the reason why the heating caloric value is or is not reduced during heating is that the initial temperature is different in each food, that is, the degree to which the food is frozen is different for type of food. Since the heat conductivity and the convection property are different in each type of food, and the steam generation starts locally in some foods, the value of K is different for every type food.
Consequently, a user of the prior art heating apparatus has been obliged to select the key most suitable for the food to be heated from among several select keys. However, since only 5-6 menus can be provided at most on the keyboard of the apparatus, the user is required to look into a cookbook or the like whenever he or she does not know whether the food which is not indicated on the keyboard is able to be automatically heated and cooked. For example, when the user wishes to reheat macaroni, the user may not be able to find out which key he or she should select. A market survey reveals, accordingly, that the automatic heating function is not utilized at a high rate although users do make use of the reheating operation at a high rate. This is because it is quite a burden for the user to select one key from among many select keys.
According to the above-described heating apparatus marketed in the first stage of development, reheating is performed through the selection of a key from among many select keys which are arranged in accordance with the kind and the initial temperature of the food to be heated. On the other hand, according to the heating apparatus marketed in the second stage of development of these types of apparatus, foods are classified into the group of frozen foods and the group of cold foods and two select keys are respectively provided for reheating frozen foods and cold foods. The reheating operation in such a heating apparatus will be described hereinbelow with reference to an operating panel of the apparatus shown in FIG. 4.
The heating apparatus is provided with a gas sensor and a weight sensor which detects the weight of the food to be heated, such as disclosed in U.S. Pat. No. 4,590,350. For heating the group of cold foods, the threshold value for detection of the gas sensor is set high, and at the same time, the heating time is calculated in accordance with the detected total weight of the food (including the packaging). The heating apparatus is so arranged that the gas sensor and the weight sensor are controlled a parallel relation. Accordingly, food having a small K value is heated on the basis of the weight detected by the weight sensor, while food having a large K value is heated on the basis of the elapsed and the amount of moisture as detected by the gas sensor.
Meanwhile, for heating the group of frozen foods with the heating apparatus, the heating caloric value is changed after the first detection point when the steam is detected as being generated from the food, and then heating is continued.
Thereafter, the ratio of the time before the second detection point when it is detected that the generated amount of steam has reached a predetermined amount with respect to the time lag between the first detection point and the second detection point is obtained. And an additional heating factor K corresponding to the calculated time ratio is obtained. When the K value is small, the food is determined to have a relatively low amount of moisture and is easy to get warm, and therefore the additional heating time K.times.T is short. On the contrary, when the K value is large, the food to be heated is regarded as full of moisture and hard to warm, and so a long additional heating time K.times.T is established.
How the heating of the cold food group is carried out in the prior art apparatus will be explained with reference to the graph of FIG. 5 which shows the amount of steam detected by the gas sensor in accordance with the lapse of the heating time. Three points with the mark * are the conventional detection points of FIG. 3, while the "reheat" point is a new detection point disclosed in U.S. Pat. No. 4,590,350. The new detection point has a considerably higher threshold value as compared with the conventional ones, and is defined approximately at the center of the conventional finishing points for "soup" and "curry/stew". "Soup" is a little hotter at the new detection point, and "curry/stew" which is in a gelled state is a little lukewarm at the new detection point, which is no inconvenience in practice use, though. On the contrary, however, "cold boiled rice" is considerably overheated at the new detection point and is turned into solid rubber-like material. Therefore, the weight sensor is employed so as to prevent cold boiled rice or some kinds of soups from being overheated. The new detection points of the gas sensor are plotted in FIG. 6 for each menu. In the above-described heating apparatus, cold boiled rice and consomme are overheated, while curry and noodles are finished in an almost favorable condition. Therefore, the overheating of the cold boiled rice and consomme is prevented in the following manner. The total weight (including the weight of a container) of the food is measured by the weight sensor, and the necessary cooking time for the food is calculated on the basis of the detected total weight of the food. The measurement of the weight by the weight sensor is controlled in parallel (by OR logic) with the detection of the amount of gas detected by the gas sensor. At this time, if a calculation formula is suitably selected, only cold boiled rice, consomme or milk that would be overheated if the heating thereof were based solely on the detection of the amount of gas by the gas sensor can be heated on the basis of the measurement of the weight by weight sensor. This is because, since cold boiled rice, consomme or milk is generally put in, for example, a rice bowl or a teacup having a large capacity (150-400 cc) in comparison with its own weight (70-200 g), the weight of the food with respect to the total weight is large. Accordingly, the detection of the amount of gas by the gas sensor when heating the cold boiled rice, consomme or milk is delayed as compared with the heating of noodles or curry/stew if they have the same total weight, and therefore the cooking time of the cold boiled rice, consomme or milk is as shown in FIG. 6. Thus, the cold boiled rice, consomme or milk can be automatically cooked on the basis of the detection of the weight thereof by the weight sensor.
From experiments, it has been found that the above-mentioned calculation formula can be expressed as a linear function To=AWo wherein constant A is preferably 0.3 or so (To (second) and Wo (g)). Cold boiled rice, consomme or milk is well heated after the weight time expressed above. Moreover, when curry, noodles or a small amount of cooked vegetables (1/2 cup) are heated over the weight time, the result is better than when they are heated on the basis of the detection of the amount of gas by the gas sensor. That is, the weight sensor is effective to improve a poor correspondence between the volume of the food and the amount of gas sensed by the gas sensor (to prevent the heating from being stopped too late).
Hereinafter, how the group of frozen foods is heated in the prior art devices will be described.
The food to be heated is started to be heated by a large amount of power as shown in FIG. 7. As the heating of the food proceeds, the power is lowered as shown in FIG. 7(a). The food is roughly heated initially with the high power output. Then, when the gas sensor detects a small amount of steam or gas generated from the food, that is, at the first detection point, the power for heating the food is switched to low. The reason why the heating power is switched from high to low is that, since the food is roughly heated initially with the high power output, only a limited part of the food is heated which suddenly discharges a great amount of steam. Therefore, a large part of the food remaining has been sufficiently heated at the second detection point. Namely, heating is interrupted earlier. Accordingly, by changing the heating power from high to low, the heat of the limited part of the food which has already been heated can be transmitted to other parts of the food. Thus, while heating is continued, the temperature of all of the food is raised, to suddenly increase the amount of steam or gas per unit time at the time detection point. When the signal level of the gas sensor at the second detection point is obtained in accordance with increase of the amount of steam generated from the food, an additional heating time K.times.T2 after the second detection point is obtained based on the ratio of the time from the start of heating to the second detection point with respect to the time lag between the first detection point and the second detection point. Heating is further continued for the additional heating time and is then stopped. During the time lag between the first detection point and the second detection point, the food which has been partly heated is entirely warmed, and accordingly, the time lag between the first detection point and the second detection point reflects the conduction speed of the heat thus representing the type of the food. On the other hand, the time from the start of heating to the second detection point indicates the entire volume of the food. Therefore, the food can be expressed as a general characteristic value indicative of the type and volume of the food by the above-mentioned time ratio. If heating is continued after the second detection point for an additional heating time corresponding to the product K.times.T2 of the additional heating time factor K which is obtained on the basis of the characteristic value and the time period T2 from the start of heating to the second detection point, the food can be heated in a manner suitable for the type and volume thereof.
The foregoing description is related to reheating of the cold food group and the frozen food group in the heating apparatus described above. As shown in FIG. 4, two select keys are alloted in the prior art heating apparatus for respectively reheating the cold foods and the frozen foods. Meanwhile, since there are frozen foods which can be eaten raw if they are only defrosted, the select keys may include both a "defrost" key and a "defrost-reheat" key. However, this brings about a dangerous possibility of an erroneous operation by the user. From the above viewpoint, one select key is desirable for reheating all groups of foods.